Interpretive Summary: Tachykinins are members of a family of peptides demonstrated to act as neurohormones in mammals and invertebrates, including insects. In conjunction with their respective receptors, these peptide hormones appear critical to a number of motor and sensory functions, particularly related to central and peripheral neuron activity and the contraction of smooth muscle. STKR is a G protein-coupled receptor that was cloned from the stable fly, Stomoxys calcitrans. Multiple sequence comparisons show that the amino acid sequence of this insect receptor displays several features that are typical for known tachykinin receptors. Functional expression of the cloned STKR was obtained in cultured insect cells and the pharmacology of STKR is analyzed by studying the effects of the most important antagonists for mammalian tachykinin-receptors on STKR-expressing insect cells. Stc-TK, an insectatachykinin that was recently discovered in the stable fly also proved to have potent interactions with STKR.

Technical Abstract:
STKR is a G protein-coupled receptor that was cloned from the stable fly, Stomoxys calcitrans. Multiple sequence comparisons show that the amino acid sequence of this insect receptor displays several features that are typical for tachykinin (or neurokinin, NK) receptors. Insect tachykinin-related peptides, also referred to as "insectatachykinins", produce dose-dependent calcium responses in Drosophila melanogaster Schneider 2 cells, which are stably transfected with this receptor (S2-STKR). These responses do not depend on the presence of extracellular Ca(2+) -ions. A rapid agonist-induced increase of inositol 1,4,5- trisphosphate (IP3) is observed. This indicates that the agonist-induced cytosolic Ca(2+) -rise is caused by a release of Ca(2+) -ions from intracellular calcium stores. The pharmacology of STKR is analyzed by studying the effects of the most important antagonists for mammalian NK-receptors on STKR-expressing insect cells. The results show that spantide II, a potent substance P antagonist, is a real antagonist of insectatachykinins on STKR. We have also tested the activity of a variety of natural insectatachykinin analogs by microscopic image analysis of calcium responses in S2-STKR cells. At a concentration of 1 micro M, almost all natural analogs produce a significant calcium rise in stable S2-STKR cells. Interestingly, Stc-TK, an insectatachykinin that was recently discovered in the stable fly (S. calcitrans), also proved to be an STKR-agonist. Stc-TK, a potential physiological ligand for STKR, contains an Ala-residue (or A) instead of a highly conserved Gly- residue (or G).